An electrode furnace (EF) enables rapid heating of a sample material used to create gases. These gases are then analyzed for their composition using a variety of scientific methods. The EF operates by generating a high current which is passed through a graphite crucible using an electrode. The current heats the crucible and any sample material therein in excess of three thousand degrees Celsius (3000° C.).
Prior art systems have used large mains-frequency (50 Hz-60 Hz) power supplies to generate the high currents necessary to rapidly produce enough heat to drive off gases in the sample material. These types of linear power supplies require a large iron core transformer making them bulky and difficult to integrate into the EF. Although higher frequency switching supplies can be used for reducing the transformer size, these types of switching supplies often have problems when delivering a high current to the load. This is primarily due to the stray and/or residual inductance created by the braided or other flexible lead wire used in connection with the electrode, the electrode inductance, and the transformer leakage inductance. The stray inductance results in a impedance that increases with frequency and is in series with the crucible resistance. At typical mains input frequencies of 50 Hz-60 Hz, the stray inductance contributes an insignificant amount of inductive reactance to the system. Therefore, the transformer secondary circuit impedance is dominated by the crucible resistance at 50 Hz-60 Hz. At switching frequencies normally utilized by switching power supplies, the inductive reactance created by the stray inductance can be many times that of the crucible resistance.
FIG. 1 is a block diagram illustrating a prior art EF system 100 using a phase chopper supply. As described herein, the EF system 100 is used for heating a crucible 109. A mains input voltage 101 is supplied to a phase controlled chopper 103 used to regulate the output current of a step down transformer 105. The transformer 105 works to supply a substantially high current through a flexible connection 107 to a crucible 109. The crucible 109 is used for holding analytical samples. The flexible connection 107 consists of the secondary circuit leads and the electrodes used to hold the crucible 109. Because the phase controlled chopper 103 only conducts during a portion of the mains input 101 alternating cycle, the phase controlled chopper 103 heavily loads the mains input voltage 101 by drawing large amounts of non-sinusoidal current. This often results in voltage disturbances to other devices connected to the same mains supply. Moreover, the non-sinusoidal current drain creates a poor power factor that increases the apparent power required to operate these devices.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.